Tag Archives: greek astronomy

We measure time based on motions in space. The Earth rotates on its axis once a day. The Moon orbits the Earth about once a month. The Earth orbits the Sun once a year. That leaves the week as the only aspect of our calendar not directly tied to the Earth, Moon, or Sun. The week, as it turns out, is based on the other planets of our solar system–at least, those easily visible to the naked eye.

Early astronomers were able to distinguish planets from stars because planets seem to move against the starry background. The stars are always rising, moving across the sky, and setting due to Earth’s rotation. They seem to form the same patterns all the time; we never see them move relative to each other. (In fact the stars do have proper motion, but we don’t notice it over a time frame as short as a human life or even over several generations). Anything shifting noticeably over several days was a ‘wandering star’, or planet. Early astronomers identified seven ‘wanderers’: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, and the Greeks placed them in just that order.

This order, of course, is wrong; it makes the basic error of putting the Sun in orbit around the Earth when in fact the Earth orbits the Sun. Fixing this error by replacing the Sun with the Earth, however, makes the order from Mercury to Saturn correct. That’s because the order is based on something directly observable–the planets seem to move among the background stars at different rates. Ancient observers saw the Moon reappear near the same set of stars once a month. Saturn, on the other hand, takes 29.5 years to reappear in the same part of the sky.

The different speeds are even more apparent when two or more planets are near one another in the sky (an alignment called conjunction). Any planet in conjunction with Saturn catches up to Saturn and then passes it. It’s never the other way around. Any planet (other than Saturn) in conjunction with Jupiter catches and passes Jupiter, never the other way around. For early astronomers, slowness was associated with distance. By carefully observing the planets’ motions and planetary conjunctions, early observers could place them in order.

Ancient Roman writer Dio Cassius was among the first to explain how the order of the planets from slowest to fastest (and thus from outside in) generated the week. The system involves the 24-hour day and an astrological belief that each hour was ‘ruled’ by a planet following the order above, such that Saturn’s hour was followed by Jupiter’s, then Mars’, then the Sun’s, and so on. Further, whichever planet governed the first hour of each day governed that whole day. On Saturn’s day, then, the hours were as follows:

Since there are 24 hours in a day, the 25th hour of Saturn’s day is the first hour of the next day. Therefore, Saturn-day is followed by Sun-day. Redo the list of hours, this time starting with the Sun, such that hours 1, 8, 15, and 22 are the Sun’s. Hour 25 becomes the Moon’s hour, which means the Sun-day is followed by Moon-day. Repeat the list with the Moon in first position, and eventually the following order of days emerges:

If Venus governs the first hour, Saturn governs the 25th, and the cycle begins again. A full table of the hours and days is here (this list also has the name of the days in 30 different languages).

You probably recognize Saturday, Sunday, and Monday in this list. To get the other English day names from this list, we have to translate by replacing the planet names, which are names of Roman deities, with roughly equivalent Germanic deities. Languages derived directly from Latin have preserved the Roman gods’ (thus the planets’) names more faithfully. For example, you can recognize Latin luna (the Moon) in French lundi, Spanish lunes, and Italian lunedì.

Similarly, Mars-day is martes in Spanish, mardi in french, and martedì in Italian. Germanic tribes, however, replaced the Roman war god Mars with their own warlike god Tiw (or Tyr for the Norse). Thus, Mars’ day became Tiw’s day or Tuesday.

‘Mercury-day’ is recognizable in French mercredi, Spanish miércoles, and Italian mercoledì. The Germanic pantheon had no messenger god that corresponded well to the Roman Mercury, so they equated him with Woden (Norse Odin). Both Woden and Mercury were gods who escorted the recently deceased to the underworld. Also, Woden became the fastest god when he rode his eight-legged horse Sleipnir.

Jupiter’s original name in Latin was Jovis (‘Jove’ to English writers); the name Jupiter is a contraction of Jovispater (‘father Jove’). ‘Jove-day’ is recognizable in French jeudi, Spanish jueves, and Italian giovedì. Although Jupiter, like the Greek Zeus, was the king of all the gods, his actual domain was the weather. In particular, he was the god who caused storms and struck people with lightning. Thus Germanic tribes assigned his day to Thor, their god of thunder. Thor’s day is Thursday.

‘Venus-day’ is still recognizeable in French vendredi, Spanish viernes, and Italian venerdì. Germanic tribes replaced Venus’s name with that of Frigg, the wife of Woden who was associated with married women and whom they called upon to help in giving birth. Frigg-day is Friday.

As the Germanic tribes had no one in their pantheon who even roughly corresponded to Saturn, Saturn’s name remains in Saturday. Ironically, the Latin-based languages have lost ‘Saturn-day’ as the day’s name. Spanish sábado and Italian sabato derive from the word ‘sabbath’ (as does French samedi, through a more complex etymology). This is due to the influence of the Catholic Church, which was loath to name the days of the week after pagan gods, and sought to replace the planetary names.

The Church designated Sunday ‘Lord’s Day’ (dies dominicus), called Saturday the sabbath (sabbatum), and numbered the weekdays from 2 to 6. Except in Portugal, however, the numbered weekdays never replaced the planetary days in popular usage. Everyday people in southern Europe did adopt the Church’s terms for the weekend days. Northern Europe, largely outside the influence of the Catholic Church, was less affected by this; we retain ‘Saturday’ and ‘Sunday’ in English as a result.

In November and December 2008, you can make for yourself some of the observations that helped astronomers of antiquity imagine the solar system. The two brightest points of light in the southwest tonight are Venus and Jupiter. They outshine all stars we ever see at night and are visible even in twilight. But don’t wait too late; you’ll need to look in the hours right after sundown before the two planets set. Venus, lower to the horizon, is the brighter of the two. Its closeness to us and the clouds that cover the whole surface and reflect most sunlight back into space cause Venus to outshine the much larger Jupiter.

Watch as Venus gets closer and closer to Jupiter each night this month. This is exactly how ancient astronomers could tell that Venus and Jupiter were not stars. On November 30 and December 1, watch as Venus passes 2 degrees ‘under’ Jupiter. (The crescent Moon also passes by on these nights). Imagine ancient Greek astronomers concluding that Venus is closer because it is faster. Keep watching each night in December as Venus pulls away from Jupiter, getting higher in the dusk sky while Jupiter sinks into the Sun’s glare by early January. Early astronomers would have seen this as the Sun catching up to Jupiter while Venus pulls away; observations like this account for the Sun’s position in the ancient order of ‘planets’. Of course, we now know better–the Sun’s apparent motion is really ours. Earth is going around the far side of the Sun from Jupiter’s position, putting Jupiter behind the Sun as the New Year opens.

Venus remains an evening star until March 2009. Compare Venus to the stars around it, and you’ll see it slow down and then move ‘backwards’ towards the Sun’s position each night in March. That’s because Venus will have come around to our side of the Sun, and will be passing us up on its faster orbit.

Should you make any of these observations on a Thursday or Friday, you can reflect on why those days have those names.

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.